Benteke was forced to miss the World Cup after suffering a ruptured Achilles tendon in training back in April, while Kozak broke his leg in January.
"I don't put a time limit on it because their two injuries were really bad," Sky Sports News quotes Lambert as saying.
"But Christian has started jogging outside, so he is doing well. Libor is doing great. So a few weeks I think. What will help us certainly will be the international break."
Villa will begin the new season away against Stoke City on Saturday.